Conventionally, it has been common for a resonator using coplanar lines formed on a plane circuit substrate and a filter using the same to be constituted by having a plurality of lines arranged. As a technology reducing the size of resonators and filters using these coplanar lines, there is known the technology, disclosed in Patent Reference 1, of eliminating lumped-parameter elements for coupling, and devised so that the lines forming a λ/4 resonator (λ being a wavelength) can be directly arranged in series.
In FIG. 20, an example of a filter using coplanar lines shown in Patent Reference 1. Filter 200 consists of a series connection of four λ/4 coplanar resonators Q1, Q2, Q3, and Q4 patterned by photolithography-based etch processing of a ground conductor 202 provided by means of vapor deposition or sputtering over the entire surface of a dielectric substrate 201 formed as a rectangular plate.
The four λ/4 coplanar resonators Q1, Q2, Q3, and Q4 are formed by center conductors 203, 204, 205, and 206, having an electric length corresponding to ¼ of the wavelength of the used frequency, which are formed on the center line in the longitudinal direction of rectangular plate shaped dielectric substrate 201, and ground conductor 202 formed by leaving a spacing of a gap g20 on both sides in the extended direction thereof.
One end of center conductor 203 of λ/4 coplanar resonator Q1 is connected to the grounded ground conductor 202 and has an input/output terminal P1 derived from the extension direction of center conductor 203 on one longitudinal direction side of dielectric substrate 201.
Opposite the other end of center conductor 203 forming resonator Q1 via a capacitive coupling part C1 due to a gap g21, one end of center conductor 204 forming resonator Q2 is arranged with the same width as that of center conductor 203. The other end of center conductor 204 is electrically connected to ground conductor 202 on both longitudinal direction sides of center conductor 204 by means of rectilinear line conductors 207 and 208 and forms an inductive coupling part L1. Via linear line conductors 207 and 208 which constitute this inductive coupling part L1, the other end of center conductor 204 (one end of center conductor 205) is extended as is and center conductor 205 constituting resonator Q3 is formed.
Opposite the other end of center conductor 205 forming resonator Q3 via a capacitive coupling part C2 based on a gap g22, one end of center conductor 206 forming resonator Q4 is arranged with the same width as that of center conductor 205, the other end of center conductor 206 being electrically connected to ground conductor 202 and there being derived, from an extension direction of center conductor 206, an input/output terminal P2 on one longitudinal direction side of dielectric substrate 201, so that a filter is constituted.
Patent Reference 1: Japanese Patent Application Laid Open No. 1999-220304 (FIG. 1)